Docker containers are ephemerals and they are not designed to store important data in.

Nevertheless here two methods on how to achieve this goal.

Paste it in docker-compose.yml

version: '3'
services:

  web:
    image: mitlabs/apache-php7.4
    ports:
      - 8080:80
    volumes:
      - .:/var/www/html
    links:
      - 'mysql'


  mysql:
    image: mariadb:latest
    restart: always
    volumes:
      - mariadb-volume:/var/lib/mysql
    ports:
      - "3306:3306"
    environment:
      MYSQL_ROOT_PASSWORD: example
    restart: always

  phpmyadmin:
     image: phpmyadmin/phpmyadmin
     links:
      - 'mysql:db'
     ports:
      - 8081:8081

  
  adminer:
    image: adminer
    restart: always
    ports:
      - 8082:8082


# volumes:
    # mariadb-volume:

You can use any similar container for mysql:

version: '3'
 
services:
  db:
    image: mysql:5.7
    container_name: db
    environment:
      MYSQL_ROOT_PASSWORD: my_secret_password
      MYSQL_DATABASE: app_db
      MYSQL_USER: db_user
      MYSQL_PASSWORD: db_user_pass
    ports:
      - "6033:3306"
    volumes:
      - ./data/db:/var/lib/mysql

After creating file, just run the below command to launch container.

docker-compose up -d

Output:

Creating network "db_default" with the default driver
Creating db ... done

In this case the MySQL container creats all files on host machine under ./data/db directory. To view these files, just run below command.

ll ./data/db
total 122940
drwxrwxr-x 4   999 jazio       4096 Nov 20 17:54 ./
drwxrwxr-x 3 jazio jazio       4096 Nov 20 17:52 ../
-rw-rw---- 1   999 docker     32768 Nov 20 17:54 aria_log.00000001
-rw-rw---- 1   999 docker        52 Nov 20 17:54 aria_log_control
-rw-rw---- 1   999 docker       976 Nov 20 17:54 ib_buffer_pool
-rw-rw---- 1   999 docker  12582912 Nov 20 17:54 ibdata1
-rw-rw---- 1   999 docker 100663296 Nov 20 17:54 ib_logfile0
-rw-rw---- 1   999 docker  12582912 Nov 20 17:54 ibtmp1
-rw-rw---- 1   999 docker         0 Nov 20 17:54 multi-master.info
drwx------ 2   999 docker      4096 Nov 20 17:54 mysql/
drwx------ 2   999 docker      4096 Nov 20 17:54 performance_schema/

##Data persistence

Option 1 Keep data in data volumes

Data volumes

volumes:
    mariadb-volume:

This is the recommended method (donno why) You store the mysql data inside the container.

It will persists even if you run

docker-compose down

But you can DESTROY it if you run:

docker-compose down -v
Removing network flowera-drupal_default
WARNING: Network flowera-drupal_default not found.
Removing volume flowera-drupal_mariadb-volume

Note: when changing the password make sure you comment the volumes lines. It will affect the new user and keys even if you create fresh container. Remove mount location, as it will pick a user name and password from this location, also will not run your DB init script too.

You can vew anytime the volume with docker volume ls

##Option 2 – Storing MySQL Data on Host Machine

you can keep database files on the host machine. In any case docker container get terminated, you can relaunch container using the existing data files.

Create a directory to keep your MySQL data files. I am creating below directory structure under the current directory.

  mysql:
    image: mariadb:latest
    restart: always
    volumes:
      - ./data/db:/var/lib/mysql
    ports:
      - "3306:3306"
    environment:
      MYSQL_ROOT_PASSWORD: mossad
    restart: always

  phpmyadmin:
     image: phpmyadmin/phpmyadmin
     links:
      - 'mysql:db'
     ports:
      - 8081:8081

  
  adminer:
    image: adminer
    restart: always
    ports:
      - 8082:8082

mkdir -p ./data/db

Then configure docker-compose.yml to use ./data/db as volume to store all files created by the MySQL server. Next create compose file in current directory.

docker-compose.yml:
version: '3'

services:
  db:
    image: mysql:5.7
    container_name: db
    environment:
      MYSQL_ROOT_PASSWORD: my_secret_password
      MYSQL_DATABASE: app_db
      MYSQL_USER: db_user
      MYSQL_PASSWORD: db_user_pass
    ports:
      - "6033:3306"
    volumes:
      - ./data/db:/var/lib/mysql